1. Field of the Invention
The present invention relates to various write controlling devices or color image forming apparatuses such as a copier, color printer, color facsimile, or color multi-function peripheral, etc., having a multiple-beam scanning device.
More particularly, the present invention relates to a technology for preventing color shifting (the position shifting of the toner image of the respective colors) in a sub-scanning direction.
2. Description of the Background
Conventionally, color image forming apparatuses (and methods) form a color image by using a single laser light source. In these cases, a photosensitive body moves in a sub-scanning direction by way of a rotational drive force created by the rotational driving section and is uniformly charged by a charging mechanism. One line of laser beam emitted from the single laser light source is scanned, or moved in a predetermined direction, by a scanning mechanism, such as a rotating polygon mirror, etc., and radiated to the charged surface of the photosensitive body. Using the beam, plural color images are sequentially written on the charged surface of the photosensitive body as electrostatic latent images.
The electrostatic latent images of the plural colors thus formed are developed by plural developing media and are thereby converted to respective visible toner images of plural colors. The toner images of plural colors formed on the photosensitive body are superposedly transferred respectively onto an intermediate transfer body by using a transfer mechanism. Thereby, a full-color image can be formed on the transfer mechanism.
The full-color image thus transferred onto the intermediate transfer body is further transferred onto a transfer material such as recording paper, OHP sheet, etc. and conveyed from a paper feeding apparatus by use of the (other) transfer body. The transfer material having the full-color image with the transferred image thereon is discharged from the color image forming apparatus.
Hereupon, the scanning mechanism is rotatively driven by a motor, etc. with a predetermined revolving velocity. A line synchronization signal generating mechanism detects a light beam (laser beam) from the scanning mechanism at a predetermined position and generates a line synchronization signal. The laser beam is modulated by the image signal in synchronism with the line synchronization signal, and thus the image writing operation is performed line by line.
An intermediate transfer standard signal generating mechanism detects a mark on the intermediate transfer body at a predetermined position and generates an intermediate transfer standard signal. The image forming operation of the respective colors for forming the toner images of the respective colors on the photosensitive body is performed in synchronism with the intermediate transfer standard signal.
In such a color image forming apparatus, when high efficiency is required, it is necessary to raise the revolution rate of the scanning mechanism or the frequency of the image signal. However, the revolution rate of the scanning mechanism or the frequency of the image signal are limited and the color image forming operation consequently could not always be performed with the desired speed. In order to cope with such problems, there has been proposed a color image forming apparatus in the multiple-beam system having a plurality of laser light sources.
In such a multiple-beam system color image forming apparatus, plural laser beams emitted from plural laser light rays are scanned by the scanning mechanism such as a polygon mirror, etc., and radiated to the charged surface of the photosensitive body. Thereby, the images are simultaneously written on the photosensitive body.
The line synchronization signal generating mechanism detects the light beam from the scanning mechanism at the predetermined position and generates one line synchronization signal per light beam. The plural lines of laser beams are modulated by the image signal in synchronism with the line synchronization signal, and the writing operations of the image per plural lines are performed simultaneously. Consequently, since the amount of the information to be written on the photosensitive body is increased, the revolution rate of the scanning mechanism or the frequency of the image signal can be reduced, and thereby the stable image can be formed with high speed.
On the other hand, in such multiple-beam system color image forming apparatuses, since the intermediate transfer standard signal is not synchronized with the line synchronization signal, there is a probability that, the higher the number of laser light sources become, an increase in the phase difference between the intermediate transfer standard signal and the line synchronization signal is incurred. Thereby, the starting position of writing of the image in the sub-scanning direction is largely shifted. Consequently, a color shifting (the position shifting of the toner image of the respective colors) occurs and the quality of the color image deteriorates.
In an attempt to address this defect, there has been proposed a color image forming apparatus having a multiple-beam system in the Japanese Laid-open patent application 1998-239939. This color image forming apparatus includes compensation mechanisms configured to adjust the start position of writing images of respective color components in the sub-scanning direction and thereby compensating the color shift, by changing over the light beam by first writing the images of the respective colors on the photosensitive body among the plural light beams in accordance with a phase relationship between the phase of the intermediate transfer standard signal and that of the line synchronization signal.
However, with such a conventional color image forming device, speed unevenness occurs (a) to a photosensitive body and an intermediate transferring body by the contact or estrangement between a developing roller of the developing unit and a photosensitive body, and (b) between a transferring roller of a second transferring station and an intermediate transferring body. For this reason, a color shift may still occur in a sub-scanning direction.